Eye Glossary A–Z: Eye Terms, Parts and Diseases Explained

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Illustration of an iris on a light background

This eye glossary explains 67 eye terms in plain English: the parts of the eye, the numbers on a glasses prescription, and the common and less common eye diseases. Each entry gives a short definition and links to a full page. The terms are checked by Laser Istanbul Clinic's three eye surgeons, Prof. Dr. Ahmet Satıcı, Doç. Dr. Muhammet Derda Özer and Op. Dr. Aysan Bayram. A glossary explains words; only an eye examination can tell you what is happening in your own eyes.

Eye reports and prescriptions are full of words that nobody explains: sphere, axis, macula, zonules, uveitis. This eye glossary puts those words in one place. Every term has a one-line definition below and its own page with the full explanation, so you can read your results, prepare for an appointment or simply understand what your doctor said.

The glossary is organised into eight groups instead of one long alphabetical list, because most people arrive with a question rather than a letter: What do the numbers on my prescription mean? Where is the macula? Is a lump on my eyelid a stye? Use the group headings to find the area you need, then open the term page.

A note on the Turkish titles. "Prof. Dr." means Professor Doctor. "Doç. Dr." means Associate Professor (Doçent) Doctor, and "Op. Dr." means Operator Doctor, the title of a specialist surgeon in Türkiye. Where a group below relates to a treatment, we name the surgeon who leads it.

This page is for learning, not self-diagnosis. Many eye diseases look alike and some, such as retinal detachment, need treatment within hours. If you have sudden changes in your sight, use the "when to act fast" table further down and seek care where you are now.

Knowledge centre: all eye health guides

Eye terms by group

Reading your prescription and how the eye focuses

How strong your glasses are, why reading gets harder after 40, and the lens types that correct it. Refractive questions (glasses, laser and lens surgery) are handled by Prof. Dr. Ahmet Satıcı and Op. Dr. Aysan Bayram, who both perform laser vision correction.

Laser eye surgery guides

Parts of the eye (anatomy)

The structures light passes through, from the clear front window to the nerve that carries the picture to the brain.

Eye pressure, glaucoma and cataract

Cataract is a clouded lens; glaucoma is damage to the optic nerve. Cataract is treated by all three of our surgeons; glaucoma is led by Prof. Dr. Ahmet Satıcı.

Lens and cataract guides

Cornea and the surface of the eye

Conditions of the clear front window and the membrane that covers the white of the eye. Cornea conditions are led by Doç. Dr. Muhammet Derda Özer.

Keratoconus and cornea guides

Eyelid conditions

Lumps, bumps and position problems of the eyelids. Eyelid and oculoplastic care is led by Prof. Dr. Ahmet Satıcı.

Eyelid and cosmetic guides

Retina, uvea and optic nerve

Conditions at the back of the eye and in the eye's inner layers. Retina and uveitis care is led by Doç. Dr. Muhammet Derda Özer, with Op. Dr. Aysan Bayram for retinal disease.

Retina and glaucoma guides

Eye movement, alignment and how the eyes develop

How the eyes point, move and develop, from childhood vision problems to differences present from birth. Squint (strabismus) surgery is led by Prof. Dr. Ahmet Satıcı.

Symptoms people search for

When to act fast: signs that should not wait for an appointment

This glossary is not an emergency guide, but a few eye problems cannot wait. If you have any of these, seek urgent eye care where you are now, not a trip abroad.

If you notice…It may be…What to do
A sudden shower of floaters, flashes of light, or a dark curtain across your visionRetinal detachmentSame-day emergency eye care. The NHS says to contact NHS 111 immediately; elsewhere, go to an eye emergency service or hospital.
Sudden, painless loss of vision in one eyeEye strokeEmergency care at once. Treatment works best within hours.
A painful red eye with blurred vision or light sensitivityKeratitis or uveitisUrgent same-day assessment by an eye doctor.
New double visionDiplopiaPrompt assessment; it can have causes outside the eye.

For long-term problems (a slowly worsening prescription, blurred reading vision, cloudy or dim sight, a lump on the eyelid), the term pages explain what to expect and what an examination looks at.

Go deeper by topic

In-depth explainers that sit behind the glossary terms:

Questions about eye terms and eye diseases

An eye glossary is an A–Z style reference that explains eye terms in plain language: the parts of the eye, the words on a prescription or clinic report, and the names of eye diseases. Ours has 67 entries, each with a short definition on this page and a full page of its own. It helps you understand your results, but it does not replace an eye examination.

Eye problems that most people meet in their lifetime include refractive errors such as short-sightedness and astigmatism, cataract, dry eye, conjunctivitis, blepharitis and presbyopia. Glaucoma, macular degeneration and diabetic retinopathy matter most for long-term sight. Each has an entry in the groups above: cataract, glaucoma, dry eye syndrome, conjunctivitis and presbyopia.

There is no official list of ten, but these ten are the ones people ask about most, and all are explained here: cataract, glaucoma, macular degeneration, diabetic retinopathy, dry eye syndrome, conjunctivitis, keratoconus, retinal detachment, blepharitis and uveitis.

There is no official ranking, but the eye diseases that most threaten sight when untreated are the ones that damage the retina, optic nerve or cornea. Cataract is among the leading causes of blindness worldwide, according to the World Health Organization, yet it is treatable with surgery. Glaucoma has no cure, but the NHS says treatment can slow the damage; retinal detachment and eye stroke are emergencies; diabetic retinopathy and macular degeneration can cause serious sight loss. Early diagnosis changes the outcome in most of them.

Rare eye diseases affect only a small share of people, and there are many of them, so no single page can list them all. In this glossary, less common conditions include retinitis pigmentosa, Behçet's eye disease, optic neuritis, coloboma and nystagmus. If you have been given a rare diagnosis, an expert second opinion is worth having.

A prescription has up to three main numbers per eye. Sphere (SPH) is the main power, written in dioptres: minus for short-sightedness, plus for long-sightedness. Cylinder (CYL) corrects astigmatism, and axis gives its direction in degrees. Reading glasses add a further "add" power for presbyopia.

Not reliably. Pictures can point you in a direction, but many eye conditions look alike: a stye and a chalazion are both eyelid lumps, and a pinguecula and a pterygium are both growths on the white of the eye, yet they are managed differently. Use the definitions here to understand the terms, and let an eye examination tell you which one you have.

Between them, our three surgeons treat cataract, lens and laser vision correction, cornea and keratoconus, retina and uveitis, glaucoma, eyelid conditions and squint. Prof. Dr. Ahmet Satıcı leads laser, lens, glaucoma, eyelid and squint surgery; Doç. Dr. Muhammet Derda Özer leads cornea, keratoconus, retina and uveitis care; Op. Dr. Aysan Bayram treats cataract, laser and retinal disease. Not every term in this glossary needs surgery: many are explained simply so you know what you are looking at.

Knowledge centre: all eye health guides
Sources for the facts on this page
  1. American Academy of Ophthalmology, "Parts of the Eye" (anatomy functions: cornea, sclera, conjunctiva, iris and pupil, lens, zonules, vitreous, retina, macula, optic nerve). https://www.aao.org/eye-health/anatomy/parts-of-eye
  2. National Eye Institute, "How the eyes work" (cornea bends light; iris and pupil; lens; retina; optic nerve). https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/how-eyes-work
  3. American Academy of Ophthalmology, "What Is Presbyopia?": loss of close focus; most people notice it shortly after 40. https://www.aao.org/eye-health/diseases/what-is-presbyopia
  4. American Academy of Ophthalmology, "Ocular Hypertension": eye pressure higher than normal with a normal-looking optic nerve; raises glaucoma risk but is not glaucoma. https://www.aao.org/eye-health/diseases/ocular-hypertension
  5. NHS, "Cataracts": cloudy lens, blurred vision, surgery under local anaesthetic. https://www.nhs.uk/conditions/cataracts/
  6. Cambridge University Hospitals NHS Foundation Trust, "YAG laser posterior capsulotomy following cataract surgery": posterior capsule opacification, quick painless YAG laser treatment. https://www.cuh.nhs.uk/patient-information/yag-laser-posterior-capsulotomy-following-cataract-surgery/
  7. Leeds Teaching Hospitals NHS Trust, "Keratoconus & corneal cross-linking": keratoconus usually starts in the teenage years; cross-linking stabilises it. https://www.leedsth.nhs.uk/patients/resources/keratoconus-corneal-cross-linking/
  8. NHS, "Conjunctivitis": infection or allergy; infectious forms contagious, allergic not. https://www.nhs.uk/conditions/conjunctivitis/
  9. NHS, "Blepharitis": swollen, itchy eyelids; not contagious; often comes and goes. https://www.nhs.uk/conditions/blepharitis/
  10. NHS, "Stye": a small, painful lump; a hard, not very painful lump is more likely a chalazion. https://www.nhs.uk/conditions/stye/
  11. NHS, "Dry eyes": not enough tears or tears evaporating too quickly. https://www.nhs.uk/conditions/dry-eyes/
  12. NHS, "Retinal detachment": floaters, flashes, dark curtain; contact NHS 111 immediately. https://www.nhs.uk/conditions/retinal-detachment/
  13. American Academy of Ophthalmology, "What Are Floaters and Flashes?": floaters are shadows from clumps in the vitreous; sudden new floaters, a shadow or curtain need urgent care; jagged lines may be migraine. https://www.aao.org/eye-health/diseases/what-are-floaters-flashes
  14. NHS, "Age-related macular degeneration (AMD)": affects central vision; does not cause total blindness. https://www.nhs.uk/conditions/age-related-macular-degeneration-amd/
  15. NHS, "Diabetic retinopathy": caused by diabetes; early detection and treatment reduces risk of sight loss. https://www.nhs.uk/conditions/diabetic-retinopathy/
  16. American Academy of Ophthalmology, "What Is a Retinal Artery Occlusion?": sudden, painless vision loss; emergency; treatment most effective within hours. https://www.aao.org/eye-health/diseases/stroke-affecting-eye
  17. American Academy of Ophthalmology, "What Is Uveitis?": inflammation of the uvea; red eye, light sensitivity, blurred vision. https://www.aao.org/eye-health/diseases/what-is-uveitis
  18. NHS, "Lazy eye": vision in one eye does not develop properly; treatment ideally before age 7. https://www.nhs.uk/conditions/lazy-eye/
  19. American Academy of Ophthalmology, "What Is Adult Strabismus?": eyes not lined up; adults often see double. https://www.aao.org/eye-health/diseases/what-is-strabismus
  20. National Eye Institute, "Color blindness": about 1 in 12 men have colour vision deficiency; mostly inherited. https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/color-blindness
  21. World Health Organization fact sheet, "Blindness and vision impairment" (10 Feb 2026): cataract among the leading causes of blindness. https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment
  22. Definitions without a page opened in this session (standard ophthalmic definitions, to be checked by the reviewing surgeon): sphere, cylinder, axis, dioptre, anisometropia, accommodation, visual acuity, stereopsis, contact lens types, orthokeratology, limbus, tear film, rhodopsin, eye colour, keratitis, corneal erosion, pterygium and pinguecula, subconjunctival haemorrhage, chalazion cause, ptosis, entropion and ectropion, xanthelasma, retinitis pigmentosa, optic neuritis, Behçet's, ocular migraine, nystagmus, diplopia, heterochromia, exophthalmos, coloboma, photophobia, halos and glare. Search results seen for dioptre (reciprocal metre, https://en.wikipedia.org/wiki/Dioptre) and pterygium vs pinguecula (pterygium grows over the cornea, pinguecula does not); re-confirm against AAO/EyeWiki on the child pages.

Found your term? Ask a surgeon about your own eyes

Send your prescription, scans or eye reports. Prof. Dr. Satıcı, Doç. Dr. Özer or Op. Dr. Bayram will explain what they mean for your eyes and which treatments, if any, may suit you, in writing and with no obligation.

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  • Prof. Dr. Ahmet SatıcıProf. Dr. Ahmet Satıcı
  • Doç. Dr. Muhammet Derda ÖzerDoç. Dr. Muhammet Derda Özer
  • Op. Dr. Aysan BayramOp. Dr. Aysan Bayram